A bearing device for a wheel (HUB) is a machine element for rotatably supporting a tire (wheel) with a vehicle body. Also, a Constant Velocity Joint (CVJ) is a torque transmission unit for transmitting the rotation from an engine to a wheel. The HUB and the Constant Velocity Joint have the following manufacture problems:
(1) A stepped shape requiring a high working ratio is included.
(2) A presence of rolling contact surfaces requires portions with high hardness after high-frequency induction hardening process. For this reason, steel having a high carbon content such as S53C or SAE1050 is used, thereby increasing deformation resistance in the working process.
In view of the above-mentioned situation, cold forging and warm forging are difficult, thereby hot forging is widely used. In the case of hot forging, the forging temperature reaches not lower than 1000° C., and therefore the micro structure after forging is coarsened, and air cooling from the high temperature results in high hardness. Taking machinability and cold plastic workability in a subsequent process into consideration, therefore, heat treatment such as normalizing or annealing (softening heat treatment) is carried out to improve the workability in accordance with the working conditions.
The aforementioned heat treatment to improve the workability, however, requires a great amount of time and energy, hampers an in-line process and deteriorates manufacture efficiency. In view of this, a method for eliminating the softening heat treatment after forging by reducing hardness increase after hot forging by an alloy design of steel (S1) has been disclosed (Japanese Patent Laying-Open No. 06-057324 (patent document 1)). Also, steel has been proposed to improve the machinability and the strength by adjusting a composition, especially by increasing the Si content (S2) (Japanese Patent Laying-Open No. 2002-332535 (Patent Document 2)).
Patent Document 1: Japanese Patent Laying-Open No. 06-057324
Patent Document 2: Japanese Patent Laying-Open No. 2002-332535